In the case of a photoelectric conversion device used for a digital camera and camcorder, a pixel is miniaturized through decrease of the device in size and change of the device to multiple pixels. In connection with miniaturization of the pixel, the area of the light receiving portion of the photoelectric conversion element is decreased and incident light quantity is decreased, thereby the sensitivity is deteriorated.
To improve deterioration of the sensitivity, deterioration of the sensitivity is reduced by forming an on-chip microlens on the light receiving portion of the photoelectric conversion device and condensing light to the light receiving portion. Moreover, in recent years, a configuration is known in which an optical waveguide for condensing light by using light reflection is formed between an on-chip microlens and a photoelectric conversion element.
A method for fabricating an optical waveguide of the above photoelectric conversion device has a step of filling a well-shaped portion after a step of forming a well-shaped portion on an insulating layer in general. A material for embedding uses a material having a high refractive index compared to that of the insulating layer. Then, light is reflected from interfaces due to the refractive index difference between the insulating layer and the high refractive index portion and condensed.
However, as pixels are further miniaturized, the aspect ratio of a well-shaped portion becomes high and a void may be produced in the well-shaped portion in an embedding step. Particularly, this problem easily occurs in a CMOS image sensor having multilayer wiring.
Therefore, there is the following technique to solve the problem in the embedding step. Japanese Patent Application Laid-Open No. 2004-193500 discloses a method for forming an optical waveguide by dividing it into a plurality of layers having different diameters.
By forming an optical waveguide constituted of a plurality of layers for each layer, it is possible to preferably fill a high refractive index portion without producing a void. Specifically, by repeating a step of filling the inside of a well-shaped portion after a step of forming a well-shaped portion, an optical waveguide free from void is formed.
Moreover, in the case of a structure having a plurality of optical waveguides, the diameter of an optical waveguide at the light incoming side. Thereby, filling of the inside and introduction of light are made easy.
Furthermore, in Japanese Patent Application Laid-Open No. 2000-150845, it is described to form an etching stop film in order to make the depth of a well-shaped structure constant when forming a well-shaped portion.
Furthermore, Japanese Patent Application Laid-Open No. 2002-359363 discloses a structure having a light guide layer in a CCD-type photoelectric conversion device. That is, a light transmission film is formed and then, the side wall of the light transmission film is surrounded by a reflection film. The shape of the light transmission film is tapered toward a light receiving portion.
However, in the case of a structure having a plurality of optical waveguides, when light is reflected at an upper-layer optical waveguide and enters a lower-layer optical waveguide, a case of not satisfying a reflection condition occurs. Then, the light not entering the photoelectric conversion element becomes color mixture or noise component.
Moreover, when bringing a lower-layer optical waveguide into a vertical shape in a structure having a plurality of optical waveguides, light not satisfying a reflection condition depending on an incident angle may be present even for the light not reflecting from the interface of an upper-layer optical waveguide but directly entering the interface of a lower-layer optical waveguide.
Moreover, in the case of a light guide layer of a CCD-type photoelectric conversion device described in Japanese Patent Application Laid-Open No. 2002-359363, incident light quantity may be decreased because the area at the light incoming side of a light transmission film is decreased. Furthermore, light may be reflected by the light shielding film of a transfer electrode instead of entering a photoelectric conversion element.